This is an ongoing problem for me. I am working on an 83 Olds Toronado. The rear brakes were not working well, and the emergency brake didn't work at all. This car has rear disc brakes with the emergency brake lever built into the caliper. This car has had the master cylinder replaced, and the rear calipers, but now I can't get any decent fluid pressure to the rear brakes. I've deducted that the proportioning valve is stuck. I've used suction to pull fluid to the calipers, and manually bled the brakes until I'm blue in the face. The master cylinder was bench bled properly and even returned and replaced with another unit, but the same problem persists, I get a dribble of fluid to the back, but not any good hydraulic pressure. Any ideas? Is this the proportioning valve giving me fits, I almost think it has to be, but I suppose there could be another restriction somewhere, but I don't think so.